In recent years, glasses exhibiting fluorescence in the visible spectrum range generated by ultraviolet excitation have become the object of attention, and lights, displays, designed glassware, etc. prepared using such glasses are being used in practice.
The following glasses are known as glasses having a fluorescent property when excited by ultraviolet rays; (1) a fluorophosphate fluorescent glass that contains terbium (Tb) or europium (Eu) as a fluorescent agent, and may further contain another rare earth element (Patent Document 1); (2) a visible fluorescent fluorophosphate glass exhibiting blue or white fluorescence comprising phosphorus (P), oxygen (O), and fluorine (F) as a glass constituent, and at least one member selected from divalent europium (Eu), terbium (Tb), and (samarium+manganese) as a fluorescent agent (Patent Document 2); (3) an oxide fluorescent glass containing silicon (Si), boron (B), and oxygen (O) as a glass constituent, and terbium (Tb) or europium (Eu) as a fluorescent agent (Patent Document 3); (4) a blue fluorescent glass containing Cu+ ions as a fluorescent agent (Patent Document 4); (5) a fluorescent glass containing oxy nitride (Patent Document 5); etc.
However, glass containing a rare earth is expensive, and therefore, it is impractical to form the glass into a large plate having a size in the order of several tens of centimeters.
To produce a Cu-containing blue fluorescent glass, glass-forming materials are melted in a reducing atmosphere to prevent the formation of Cu2+ ions, thus giving stabilized Cu+ ions. However, in the glass production process, too much reduction causes deposition of a metallic colloid and it is difficult to obtain a glass having the desired properties.
Since an oxy-nitride-containing glass has a high melting point, it requires high temperatures of around 1700° C. and specialized equipment to produce a glass. Further, the glass has the drawback of low mechanical-processing properties.
When a glass material is utilized in products other than designed glassware, for example, in displays, lamps, built-in illumination equipment, etc., it would be desirable that the glass itself exhibits a white light emitted by ultraviolet excitation rather than exhibiting one specific color of light such as blue. However, all of the glasses mentioned above utilize a luminescence caused by transition between specific energy levels of rare earths or transition metals, and thus, only glasses that exhibit one specific color of light are obtained. Patent Document 2 discloses the composition of a glass exhibiting a white light; however, said glass requires a large amount of expensive rare earth metals as described above and further needs to be melted in a reducing atmosphere to maintain a valence balance. Therefore, the glass obtained will be extremely expensive and thus is not suitable for mass production.
Therefore, a demand exists for the development of a novel glass that contains inexpensive metal ions as a dopant and can be produced by being melted using a general method, as well as provides a white light having sufficient intensity.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 1996-133780
Patent Document 2: Japanese Unexamined Patent Application Publication No. 1998-167755
Patent Document 3: Japanese Unexamined Patent Application Publication No. 1997-202642
Patent Document 4: Japanese Unexamined Patent Application Publication No. 1998-236843
Patent Document 5: Japanese Unexamined Patent Application Publication No. 2001-214162